Newt Gingrich, the fiery former congressional leader, has scored a huge win over Mitt Romney in South Carolina's Republican primary, opening up the race to choose a challenger to US President Barack Obama.
The commanding victory allows Mr Gingrich to claim the mantle as the alternative to Mr Romney, the longtime frontrunner who has failed to persuade many Republicans that he is an authentic conservative.
Mr Romney, a former Massachusetts governor, had looked for a South Carolina victory to make him the all-but-inevitable Republican nominee. Instead, the Gingrich victory raises the prospect the race could drag out for months.
Mr Romney had benefited in earlier contests from having the conservative vote divided among Mr Gingrich, Rick Santorum and Rick Perry. But with Mr Perry, the Texas governor, dropping out on Thursday and Mr Santorum finishing a distant third in South Carolina, Mr Gingrich will look to coalesce the conservative vote ahead of the next contest, the January 31 primary in the big battleground state of Florida.
If the race boils down to Mr Gingrich and Mr Romney, it would be a fire-and-ice match-up: Mr Gingrich, a passionate, spontaneous, self-styled visionary; Mr Romney, a clean-cut, cool, methodical former CEO.
Mr Obama and his fellow Democrats would relish a tough, drawn-out Republican primary fight that ultimately weakens the nominee. Democrats see Mr Romney as the toughest potential opponent for Mr Obama, whose re-election prospects have been damaged by the sluggish US economy.
Mr Gingrich's campaign still faces many obstacles. While South Carolina's conservative electorate was a good fit for Mr Gingrich, who is from neighbouring Georgia, other states could be more difficult. Mr Gingrich lacks Mr Romney's money and organisation, and he has more baggage than other contenders, including admitted extramarital affairs, two divorces, an ethics reprimand when he was speaker of the House of Representatives, and questions about his post-Congress business dealings.
But he has been underestimated before. Twice his campaign was considered effectively dead, yet he has rebounded to win what many consider the most important of the first three primaries. And while much of the Republican establishment is wary of him, his bashing of "elites" can resonate with the Republican base.
In his victory speech, Mr Gingrich urged supporters to donate and get involved. "We don't have the kind of money that at least one of the candidates has," he said in a reference to Mr Romney. "And we proved here in South Carolina that people powered with the right ideas beats big money."
Returns from 95% of the state's precincts showed Mr Gingrich with 41% of the vote to 27% for Mr Romney. Mr Santorum, a former senator, was on 17%, and Texas congressman Ron Paul was on 13%.